Observational data accumulated over the past decade have considerably narrowed down the choice of proposals for grain models. Mixtures of particles composed of refractory species such as graphite, silicates, quartz and iron appear likely. Such particles may form in a wide range of astronomical situations, for example, cool stars, protostars and supernovae explosions. Dust grains are responsible for infrared radiation in galactic sources, and also play an important role in the synthesis of interstellar molecules.